The long-term objectives are to understand the neuroendocrine basis of anovulation in aging rats especially with regard to operation of hypothalamic peptidergic circuits that regulate the preovulatory LH surge. In the female rat, regular cyclicity is maintained by a precise timing in the preovulatory surges of LHRH and LH evoked by the timely, sequential unfolding of inhibitory (EOP and other) and excitatory (NPY and other) sequelae within the hypothalamus. Since neuropeptide Y (NPY) release is obligatory in the induction of LHRH and LH surges in young rats, we hypothesize that a progressive, age-related deficit in either NPY signal output and/or a derangement in the relationship of the NPY system with the neural clock-opioid link upstream, and the LHRH-LH axis downstream, results in disappearance of LHRH-LH surges in senescent rats. To test our hypothesis, rats of three age groups (young, middle-aged and old) will be studied with the following specific aims: 1) test that NPY neurosecretion is altered during reproductive senescence, especially in association with a loss of the preovulatory LH surge, 2) evaluate the cause of disruption (neurosecretion) in the NPY system by examining whether NPY system in old rats retains or loses the capacity to respond normally to peripheral signals (steroids) and to neuronal systems that impinge upon it (opioids) and 3) test that a progressive shift in the post-junctional excitatory action of NPY (interactions between NPY and NE on LHRH and LH release) contributes to the loss of LH surges in aged rats. NPY neurosecretion will be evaluated by assessing the signal output (in vivo and in vitro release) and the amount of signal available for release (peptide content) and synthesis (preproNPY mRNA). In vivo release of NPY will be measured by push-pull cannula perfusion from the media basal hypothalamus (MBH) and anterior pituitary (PIT) and in vitro by hypothalamic (median eminence- arcuate nucleus) incubation. Peptide content in brain nuclei and preproNPY mRNA in the MBH will be studied on a diurnal basis. Neuropeptides (NPY, LHRH), LH and steroid (E2) will be measured by RIA. Interactions of NPY and NE on LH release will be studied in vivo and interactions of NPY and NE on LHRH release will be studied in vivo by the PPC method and in vitro using ME-ARC incubations. PreproNPY mRNA will be measured by solution hybridization/ nuclease protection assay. New information derived from these studies will not only further our understanding on the neuroendocrine basis of reproductive senescence, but will also serve as a framework for future elucidation of cellular and molecular mechanisms involved in the action of a discrete signal (NPY), at discrete neuroanatomical location (medial preoptic area/median eminence/PIT) for a specific function (ovulatory LH surge) in young rats.